The CLIC family consists of six human proteins (1-6) related to C. elegans Exc4, which is essential for lumen formation. We investigated the roles of CLICs as angiogenic regulators and found that CLIC1 and CLIC4 are expressed in endothelial cells and are required for proliferation, migration and lumen formation of cultured endothelial cells. We developed Clic4 knockout mice that had aberrant retinal angiogenesis. Clic1/Clic4 double mutant embryos die in utero and have defects in vascular development, suggesting Clic1 and Clic4 are functionally redundant in the endothelium. We hypothesize that murine Clic1 and Clic4 function in developmental and pathological angiogenesis. To test this hypothesis, we will delete Clic4 using endothelial specific drivers (Clic4ECKO), in the background of the Clic1-/- genotype and analyze vascular phenotypes in the embryo, postnatal retina and during hypoxia-driven retinal angiogenesis. Although Clic genes encode putative chloride channels, the molecular action of mammalian CLIC proteins is poorly defined. We recently discovered that sphingosine-1-phosphate (S1P)-driven endothelial cell migration, adhesion junction formation, and actin stress fiber formation requires CLIC1 or CLIC4. By selective CLIC knockdowns in endothelial cells, we document both overlapping and distinct endothelial functions for CLIC1 and CLIC4, highlighting S1P receptor specific activities for CLICs. Thus, we provide the first evidence that CLIC proteins function as part of a known angiogenic pathway, S1P signaling, and our functional studies link CLICs to G protein coupled receptor signaling. We propose to use in vitro endothelial studies to explore the molecular interactions between CLICs and the S1P pathway. We will determine whether endothelial cells require CLIC1 or CLIC4 to mediate S1P-driven proliferation, survival, migration, barrier formation, collagen gel invasion, and lumen formation. We hypothesize that CLIC1 and CLIC4 promote S1P1 signaling via the PI3K/RAC1 and PI3K/Akt pathways. We hypothesize that CLIC1 is uniquely required for S1P activation of the S1P2-RhoA/ROCK pathway, while CLIC4 is required for S1P activation of the S1P1- RAS/ERK pathway. We will assess binding of CLICs to S1P receptors or their downstream intracellular signaling components. S1P1 functions during sprouting angiogenesis of the retina. Endothelial specific S1P1 loss caused increased retinal sprouting and S1P1 gain-of-function decreased sprouting. By activating or inactivating S1P1 signaling in mice, using both pharmacologic and genetic methods, along with endothelial loss of Clic4 or Clic1-/- mice, we will address the hypothesis that S1P signaling requires endothelial CLICs to regulate angiogenic sprouting.